This invention relates to the fabrication of a multilayer pattern on a dielectric substrate.
A material that is to be used as the substrate for a multilayer conductor pattern, for example in a hybrid integrated circuit, should have high dimensional stability, in order to ensure that the tolerances within which the photoprocessing masks for successive layers of the multilayer conductor pattern must be aligned do not need to take account of possible changes in the dimensions of the substrate. Ceramic materials, such as alumina, have high dimensional stability and therefore have been used as the substrate for a multilayer conductor pattern. However, ceramic materials are subject to limitation in that they are not flexible and therefore cannot be used as the substrate for a flexible circuit.
Polyimide is flexible, and its electrical properties favor its use as the substrate for a conductor pattern. However, a planar sheet of polyimide has relatively poor dimensional stability, and this has previously militated against its use as the substrate for a multilayer conductor pattern because it precludes use of fine alignment tolerances between successive layers of the pattern.
A planar sheet of copper has better dimensional stability than a planar sheet of polyimide. Sheet-form polyimide with copper adhered to one or both main surfaces of the polyimide sheet is commercially available.